1. Field of the Invention
The present invention is broadly concerned with an improved assay for the determination of motile facultative anaerobic pathogens, and especially L. monocytogenes, in samples such as meat. More particularly, it is concerned with such an assay which makes use of a substance such as an oxygen-reactive enzyme (e.g., oxyrase enzyme) for enhancing the growth rate of a target pathogen in a selected growth medium, in order to materially lessen the amount of time required for completing the assay. Assays for determining the presence of L. monocytogenes in meat samples can be completed in times many hours shorter than previous standard assays.
2. Description of the Prior Art
Listeria monocytogenes is a motile facultative anaerobic pathogen which can cause various diseases in man, including meningoencephalitis, low-grade septicemia, infectious mononucleosis-like syndrome, pneumonia, endocarditis, bacterial aortic aneurysm, localized abscesses, papular or pustular cutaneous lesions, conjunctivitis and urethritis. It is known that the pathogen can be transmitted from the eating of infected meat or milk. Accordingly, because of the pernicious effects of this pathogen, and its increasing incidence in human foods, there has been a significant increase in the concern expressed about L. monocytogenes and its effects on human health.
The accepted present-day assay for L. monocytogenes is described in "FSIS Method for the Isolation and Identification of Listeria monocytogenes From Processed Meat and Poultry Products", Laboratory Communication No. 57, May 24, 1989, distributed by the USDA and others; this publication is incorporated by reference herein. Broadly speaking, the FSIS assay involves placing a sample of meat in UVM Broth followed by incubation at 30.degree. C. for 24 hours. A 0.1 ml. aliquot of the incubated mixture is then placed in 10 ml. of Fraser Broth and incubated at 30.degree. C. or 24-48 hours. If the Fraser Broth darkens, the liquid is swabbed onto a modified Oxford agar plate, which is then incubated at 35.degree. C. for 24-48 hours. The incubated plates are then examined for L. monocytogenes colonies exhibiting characteristic surrounding black zones resulting from hydrolyzed esculin. Suspected colonies of L. monocytogenes are then gently touched with an inoculation needle and are streaked for isolation onto a Horse Blood Overlay Agar plate. These plates are incubated overnight at 35.degree. C. Thereafter, the plates are examined under a fluorescent lamp and translucent colonies are then further screened and subjected to conventional confirming tests.
Generally speaking, the prior L. monocytogenes assay involves a total time of from 56-72 hours. This represents a real difficulty for the food processor, in that food otherwise ready for shipment must be held pending the completion of assay screening. Accordingly, there is a real need in the art for an effective assay for L. monocytogenes (or other motile facultative anaerobic pathogens) which can be successfully performed in a significantly shorter period of time.